Accompanying increases in mobile stations such as PHS and portable telephones in recent years, there is a heightened social demand for effective use of frequency resources. One method that responds to this demand is communication according to the spatial multiplexing method.
The spatial multiplexing method is a method in which communication is performed by multiplexing using a carrier wave of one frequency at one time, with use of an adaptive array device that is directional in both transmission and reception to form a directional pattern towards each of a plurality of mobile stations that are in different directions.
The adaptive array device is composed of a plurality of fixed antennas, and is characterized in that it adjusts the amplitude and phase of transmission and reception waves of each individual antenna to form directivity for the plurality of antennas overall.
A radio base station that connects with a plurality of mobile stations through spatial multiplexing uses an adaptive array device. In order to separate the signal of each individual mobile station from a received signal in which the signals of the plurality of mobile stations are multiplexed, the radio base station calculates a weight factor. The weight factor is the amount that the amplitude and phase of the signal received by each antenna for each mobile station are adjusted.
The radio base station compares a signal that is obtained by multiplying the signal received by each antenna by the weight factor for the particular antenna and adding the result of each multiplication together, with a reference signal, and adjusts the weight factor so that the difference between the two signals is a minimum. Signal wave pattern data corresponding to a known set bit pattern that is a part of a control channel signal or communication channel signal is used as the reference signal.
The radio base station performs this adjustment repeatedly, and by calculating the weight factor for each antenna in regard to each mobile station that is being spatially multiplexed, and multiplying each signal received by each antenna with the weight factor for the particular antenna and adding the results together, separates the signal of each of the plurality of mobile stations. In transmission, the radio base station forms a directivity pattern towards each mobile station by transmitting a signal from each antenna that is a signal multiplied with the weight factor that was calculated at reception.
Note that spatial multiplexing techniques are described in “Path Division Multiple Access (PDMA) Mobile Communication System”, The Technical Report of the Proceeding of the Institute of Electronics, Information, and Communication Engineers RCS93-84(1994-01), pp. 37-44.
The radio base station judges whether a mobile station with which it is already communicating and another mobile station from which the radio base station has newly received a request for allocation of a communication channel are a suitable combination for communication according to spatial multiplexing, in the following way.
The radio base station calculates a response vector of both the mobile stations, and calculates a correlation value between the two calculated response vectors. Here, the response vector is information about the direction of the mobile station. The response vector correspondence value is an indicator showing how close the directions of the two mobile stations are.
When the calculated correspondence value is higher than a threshold value, it is considered that it is impossible to separate the signals of both of the mobile stations by a difference in directivity patterns because the two mobile stations are in substantially the same direction. In this case, the radio base station judges the mobile stations to be an unsuitable combination for spatial multiplexing.
Furthermore, the radio base station measures the field intensity of the signal from each of the mobile stations, and calculates a ratio of the two measured field intensities.
If the calculated field intensity ratio is higher than a threshold value, it is thought that the two signals cannot be suitably separated even if an optimal directivity pattern is formed because the intensity ratio of the two signals is higher than the gain ratio of the adaptive array device. In this case, the radio base station judges the mobile stations to be an unsuitable combination for spatial multiplexing.
The radio base station, which also uses a time division multiplex system to connect with the plurality of mobile stations, judges whether spatial multiplexing is suitable in each time slot in regard to the time division multiplex when allocating a communication channel to a new mobile station and there is no space in a timeslot in time division multiplex communication. The radio base station allocates the communication channel according to spatial multiplexing in a timeslot that is not being used by the mobile station that was judged to be unsuitable for spatial multiplexing with the new mobile station.
In this way, the radio base station performs spatial multiplexing only through combinations of mobile stations that are suitable for spatial multiplexing communication. In addition, during spatial multiplexing the radio base station follows the direction of each mobile station with the directivity pattern of the adaptive array device. Accordingly, communication is performed preventing mixing of voices and maintaining suitable communication quality.
In the above-described conventional technique, in allocating a communication channel to a new mobile station through spatial multiplexing, the radio base station judges whether spatial multiplexing between a new mobile station and a mobile station that is already communicating is suitable by comparing the correlation value of the response vectors of the two mobile stations and the ratio of the field intensity of the signal received from the two mobile stations with respective threshold values, to judge whether the two mobile stations are a suitable combination for spatial multiplexing. However, in this method there are cases in which, even when the two mobile stations are judged to be a suitable combination for spatial multiplexing, calculation of the weight factor fails and a correct directivity pattern cannot be formed.
This failure occurs in the following way.
It is desirable for the reference signal to be different for each mobile station in order for the weight factor to be calculated correctly. This is because if the waveform of the reference signal is completely the same for a plurality of mobile stations and the compared timing matches completely, the reference signals will no longer be a basis on which to differentiate the mobile stations.
In reality, since each mobile station generates its own operation clock, the timing of the signal received from each mobile station is different, so it is rare the timing completely matches. The radio base station utilizes this to detect the timing of the signal received from each mobile station, and calculate the weight factor by following the detected timing and staggering the comparative timing of each mobile station relative to the reference signal.
However, since occasionally timing of reception of signals from two mobile stations is substantially the same, cases occur in which the weight factor of the mobile stations cannot be calculated correctly. With recent improvements in the accuracy of circuit elements that compose mobile stations, employment of techniques for frame synchronization between a plurality of base stations, developments in circuit technology, and so on, such cases are occurring much more frequently.
If the radio base station cannot calculate a weight factor correctly, an appropriate directivity pattern cannot be formed towards each mobile station. Consequently, a problem arises that not only can a new mobile station not begin communication, but mobile stations already communicating may experience interruptions, and users cannot be assured appropriate communication quality.